Compaq RAID Array 4100 User Manual page 134

E-8 Compaq StorageWorks RAID Array 4100 User Guide
Data Guarding (RAID 4)
Data guarding, also called RAID 4, assures data reliability while using only a
small percentage of the logical drive storage capacity. A designated, single
drive contains parity data. If a drive fails, the controller uses the data on the
parity drive and the data on the remaining drives to reconstruct data from the
failed drive. This allows the system to continue operating with slightly reduced
performance until the failed drive is replaced.
Data guarding requires a minimum of three drives (two data drives and one
parity drive) in an array and allows a maximum of 12 drives (11 data drives
and one parity drive). For example, in an array containing three physical
drives, data guarding uses only 33 percent of the total logical drive storage
capacity for fault tolerance. A 12-drive configuration (11 data drives, one
parity drive) uses only 7 percent.
DATA DATA
DAZ-10-3.EPS
Figure E-6. Data guarding stores redundant data [P] on a separate drive
The RA4000 Controller supports RAID 4 for backward compatibility. Because
RAID 5 provides the same benefits with higher performance, Compaq
recommends using RAID 5 instead of RAID 4.
Drive Mirroring (RAID 1)
Drive mirroring, also called RAID 1, is the highest performance fault-
tolerance method. RAID 1 is the only option offering fault-tolerance
protection if only two drives are installed or selected for an array. Drive
mirroring creates fault tolerance by storing two sets of duplicate data on a pair
of disk drives. Therefore, RAID 1 is the most expensive fault tolerance method
because 50 percent of the drive capacity is used to store the redundant data.
DATA PARITY
P = Parity
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